The present invention relates to the filtering of the responses received by a secondary radar receiver.
It is known that a radar can be fitted out with a device called a secondary radar used to obtain, from cooperating carrier vehicles equipped with radar responders, coded information elements on the identity of the carrier and other coded information elements (pertaining to altitude, reporting of radio malfunctions, distress signals etc).
The transponders of the carrier vehicles emit responses whenever they are interrogated and, sometimes, they do so spontaneously in a mode of operation with selective addressing, called S mode operation, that can be used also for anticollision functions. Each radar fitted out with a secondary radar must therefore be provided with means enabling it to recognize those responses, among all the responses received, that are S mode responses so as to be able to eliminate them in order to keep only the responses in the other modes and achieve easier sorting out the responses to its own interrogations in these other modes. As standardized by the International Civil Aviation Organization (ICAO), a response in S mode is constituted by a train of pulses emitted at a carrier frequency of 1090 MHz.
Each train of pulses has a preamble and a message.
The preamble has four identical pulses with a duration of 0.5 .mu.s each. The first two pulses and the last two pulses are separated from each other by 0.5 .mu.s. The first pulse and the third pulse are separated from each other by 3.5 .mu.s.
The message or data block may be short or long. When it is short, the message has 56 pulses of 0.5 .mu.s each and, when it is long, it has 112. The modulation of the message is done by the position of the pulses which may be at the start or at the end of 1 .mu.s intervals.
The first of these intervals is 8 .mu.s behind the first pulse of the preamble.
The definition of the standard is shown schematically in FIG. 1. This figure also shows the tolerances as defined by the ICAO.
The need to filter the S mode responses has not yet arisen since interrogation with selective addressing is not operational to date. However, invitations of tenders for secondary radar response extractors are already stipulating a requirement of protection against the S mode responses for these responses are such as to induce numerous false detections of secondary responses which may saturate the extractor.
The known circuits carry out:
a detection of the S mode responses on the presence of the four pulses of the preamble; PA1 an elimination of all the pulses received during a period of time longer than that of a long S response.
This principle of detection is subject to a high rate of false alarms in the presence of "garbled" secondary responses and does not specify the length of the S mode response detected (64 .mu.s or 120 .mu.s). A response is said to be "garbled" when it is mixed with other responses to the point where one or more pulses of the response have apparent lengths that are different from their real lengths, owing to the overlapping of the pulses of the different responses. The "all or nothing" method of filtering notably reduces the probability of detection of the secondary responses for the elimination is not selective.
The present invention improves the detection of the secondary responses, reduces the number of false responses detected and prevents the saturation of the extractor when there are responses coming from a transponder with selective addressing (mode S).
It enables the search for and the selective elimination of the pulses belonging to an S mode response and hence the preserving of the secondary radar responses received during the period of the S mode response. The invention therefore relates, firstly, to a method and device for the detection of the S mode responses, capable of indicating the type of response (short or long), and secondly a method and a device to eliminate the pulses belonging to the S mode response. If the second device is to fulfil its role of eliminating the pulses of an S mode response while at the same time keeping the secondary radar response pulses, the search should be made under conditions where all the information received is saved. The invention is applicable whenever the pulses of a message having the characteristics of an S type response have to be sorted out.
It is located downline with respect to a device for the digitization of received video signals and a device for the detection of the leading edges of the pulses.
The devices that elaborate the different video signals are known per se and shall not be described. These are notably, as far as the invention is concerned, a signal that is conventionally called Log.SIGMA. and represents the power of the received signal and a signal that is conventionally called .sub.-- /.SIGMA. and represents the angular divergence between the point of origin of the received signal and the axis of a directional antenna with which the receiver is equipped.
A search is made for the presence, if any, of an S mode response at the end of each of the elementary time periods determined by a clock. In the preferred embodiment, this period referenced P is the same as the sampling period used for the digitization of the video signals. The detection method according to the invention is used to determine whether the detected S mode response is a short S mode response (duration of 64 .mu.s) or a long S mode response (duration of 120 .mu.s).
To this end, the pulse leading edges are memorized in a register with series input and parallel outputs. The information contained in the register is shifted at the sampling rate. The register has a number of compartments greater than the number obtained through the division of the duration of a short S mode response by the sampling period P. Preferably, the number of compartments is such that it enables the memorizing of the pulse leading edge No. (56+c) of a long S mode response (c&gt;0) while the first pulse of the preamble of this response is still present in the register. The parallel outputs of the register are connected to two correlators formed in a standard way by AND gates.
The correlation is aimed at ascertaining the simultaneous presence of the leading edges of the four pulses of the preamble, the b last pulses of an S mode response that is short and the c first pulses of a S mode response that is long. At output of the correlators, therefore, there is an information element relating to the presence of an S mode response and to its length. It is therefore possible to adapt the following part of the processing of the response thus detected, especially its filtering, to the length of the response. According to one embodiment of the invention, the filtering, namely the elimination of the pulses of the S mode responses so as to process only the other responses of the secondary radars, is done by considering not only the temporal position of each of the pulses with respect to the pulses of the preamble but also its level of power determined by the value of the signal Log.SIGMA. and its angular divergence determined by the value of the signal .DELTA./.SIGMA..
Normally, all the pulses that belong to a single response are at the same power level and have a same value of angular divergence.
It is therefore probable that a pulse that is well positioned in time but has a power level or value of angular divergence that is different from the mean of the other pulses of the response will be a pulse belonging to another response.
To establish the mean power of the S mode response, different principles ranging from the simplest to the most sophisticated may be used. These include:
the use of the pulses of the preamble only;
the use of the detected S mode pulses correlating with the established reference on the pulses of the preamble;
the application of the principle described by the French patent application No. 89 14416: Elaboration de la puissance d'une reponse mode S par analyse de l'histogrammme des valeurs des impulsions possible pendant la duree d'une reponse mode S (the corresponding U.S. Pat. No. 5,063,836, "Device For The Detection Of The Signals Of Transponders Interrogated By A Secondary Radar In The Presence Of Multiple-Path Phenomena" has been delivered on 5th Nov. 1991).
The power of each of the pulses, well positioned in time and thus capable of constituting a part of the S mode response, is compared with the mean power level of the pulses set up beforehand. If the difference power between the examined pulse and the mean level is below a previously fixed threshold, then the pulse is considered to form part of the S mode response. The value of the threshold can be adapted to take account of the mean value established. The weaker the response, the more it is subject to major variations owing to the increasing effect of the noise. It would therefore be advisable to raise the value of the comparison threshold for the low mean power values. This adjustment of the threshold could be done by successive stages.
To establish the mean value of the angular divergence, it is the mean value of angular divergence of the pulses of the preamble that is used.
Here again, there is a choice. The mean value of the angular divergence could be established by any other known method, especially by the method of the histogram described in the French patent application No. 89.14416 applied to the magnitude of angular divergence.
The value of the angular divergence of each pulse which, by virtue of its temporal position, is liable to belong to the S mode response, is compared with the mean value established on the first four pulses. If the difference between the angular divergence of the examined pulse and the mean angular divergence is below a previously fixed threshold, then the pulse is considered to form part of the S mode response.
The decision to reject a pulse may be taken solely as a function of the results of the comparison on LOG.SIGMA. or .DELTA./.SIGMA.. It can also be done cumulatively on both criteria, and in this case, it may be chosen only if it meets both criteria. In the embodiment that shall be described further below, the two criteria are used cumulatively, except when the level of Log.SIGMA. becomes lower than a fixed threshold, in which case only Log.SIGMA. is used.